Numerous spring-type biasing means for biasing a piston in a predetermined direction have heretofore been proposed. While such mechanical biasing means can be manufactured at a relatively low cost and the resultant inaccuracies in movement and alignment can be tolerated in most uses, there are some uses which are particularly demanding insofar as accuracy of movement and alignment is concerned, thus making mechanical or spring-type biasing means an illogical choice.
For example, where the piston of a valve having a slide-type closure means is biased in one direction by a mechanical biasing means and in the opposite direction by a pressurized fluid, such as gas, mechanical biasing means create numerous problems. This is particularly true when the piston movement is extremely small and the valve, as such, must operate to open and close under severe conditions, such as high pressure and with extreme accuracy, as where the valve is to be utilized at high pressures in gas or liquid chromatography. A valve suitable for use in the last mentioned service is described in U.S. Pat. No. 3,633,426. This particular valve has been highly successful for use as a sample valve, a back flush valve or a column switching valve in gas or liquid chromatography. This particular valve includes a diaphragm selectively distorted by a plurality of slide-type closure means, a set of pistons for alternately operating selected closure means, a pressurized fluid means for moving the pistons in one direction and a spring-type biasing means for biasing the pistons in the opposite direction. The biasing means inludes a plurality of Belleville spring washers. Such spring elements are relatively expensive to manufacture and numerous problems have been encountered in the manufacture thereof. The end result is that a number of such spring washers are not suitable for the particular use to which they are to be put because of the application of uneven pressure to the piston and inaccuracies of alignment, which also results in inaccurate piston movement. Further, any spring-type biasing means requires adjustment both for initial use and during use.